1. Field of the Invention
The present invention relates to digital data transmitting systems which convey digital words whose digits can have two logic levels (1, 0) as a binary digit, or three logic levels (-1, 0, 1) as a ternary digit. In the following specification, the "high"level ("H" level) indicates "1" levels, whether the levels are positive or negative.
2. Description of the Prior Art
In known digital data transmitting systems of this kind, it is often difficult to transmit a supplementary information signal without, firstly, increasing the complexity of the transmission and reception equipments and, secondly, without resorting to the use of a supplementary transmission medium, such as an auxiliary pair of coaxial cables. In a general manner, the supplementary information signal is a low-frequency wave, i.e. has a digit rate very low compared with that of the digital data signal. The supplementary information signal may be modulated by a carrier signal having a digit rate less than that of the digital data signal.
For transmitting the low-frequency supplementary information signal, the transmission arrangement embodying the invention entails a "H" level parity check. It is already known to use in repeaters a "H" level parity check method for detecting the rate of operating errors. This method is described in contributions Nos. 59-F and 54-F of the Study Commission XVIII and IV of the International Telegraph and Telephone Consultative Committee, August 1977.
This known method is particularly applicable to digital transmission systems in which the digital data signal to be transmitted is already arranged in or can be converted into a format resulting in what may be termed constant accumulated disparity signals.
The term "constant accumulated disparity" or "accumulated parity" is used to mean that over a predetermined period of time having a sufficient duration, the ratio of digits of different values is such that the signals have a substantially constant d.c. content, which may be zero in some cases. One way of achieving zero accumulated disparity signal is to have normal binary coded p.c.m. signals random scrambled at a transmitting device and descrambled at the reception device. Truly randomized signals have no d.c. content. Pseudo randomized signals can be made to have, for all practical purposes, a negligible d.c. content. The same is true of scrambled binary coded signals which are subsequently transcoded into ternary coded signals.
According to this known method, the digital data signal contains special signals the digital values of which reflect the parity of predetermined digital data signal events in the immediately preceding intervals. The special signals themselves have values which maintain the overall constant accumulated disparity of the transmitted digital data signal. The special signals are additional parity check digits which are inserted in the data signal to be transmitted. For example, the logic levels of the parity check digits must be so choosen in the absence error that the number of digits having high logic levels in the digital data signals is even over the predetermined period.